Ionized physical vapor deposition (IPVD) is a process which has particular utility in filling and lining high aspect ratio structures on silicon wafers. In ionized physical vapor deposition for deposition of thin coatings on semiconductor wafers, materials to be deposited are sputtered or otherwise vaporized from a source and then a substantial fraction of the vaporized material is converted to positive ions before reaching the wafer to be coated. This ionization is accomplished by a high-density plasma which is generated in a process gas in a vacuum chamber. The plasma may be generated by magnetically coupling RF energy through an RF powered excitation coil into the vacuum of the processing chamber. The plasma so generated is located in a region between the source and the wafer. Electrostatic forces affect the positive ions of coating material and direct them toward the various surfaces in the chamber. By applying a negative bias to the wafer, positive ions are attracted from the plasma to the wafer. Such a negative bias may either arise with the wafer electrically isolated by reason of the immersion of the wafer in a plasma or by the application of an RF voltage to the wafer. The bias causes ions of coating material to be accelerated toward the wafer so that an increased fraction of the coating material deposits onto the wafer at angles approximately normal to the wafer. This allows the deposition of metal over wafer topography including in deep and narrow holes and trenches on the wafer surface, providing good coverage of the bottom and sidewalls of such topography.
Certain IPVD systems are disclosed in U.S. patent application Ser. Nos. 08/844,751, now U.S. Pat. No. 5,878,423 Ser. No. 08/837,551 now U.S. Pat. No. 5,800,688 and Ser. No. 08/844,756 filed Apr. 21, 1997 now abandoned, assigned to the assignee of the present application. The disclosures of these applications are hereby expressly incorporated by reference herein. Such systems include a vacuum chamber which is typically cylindrical in shape and provided with part of its curved outer wall formed of a dielectric material or window. A helical electrically conducting coil is disposed outside the dielectric window and around and concentric with the chamber, with the axial extent of the coil being a significant part of the axial extent of the dielectric wall. In operation, the coil is energized from a supply of RF power through a suitable matching system. The dielectric window allows the energy from the coil to be coupled into the chamber while isolating the coil from direct contact with the plasma. The window is protected from metal coating material deposition by an arrangement of shields, typically formed of metal, which are capable of passing RF magnetic fields into the interior region of the chamber, while preventing deposition of metal onto the dielectric window that would tend to form conducting paths for circulating currents generated by these magnetic fields. Such currents are undesirable because they lead to ohmic heating and to reduction of the magnetic coupling of plasma excitation energy from the coils to the plasma. The purpose of this excitation energy is to generate high-density plasma in the interior region of the chamber. A reduction of coupling causes the plasma density to be reduced and process results to deteriorate.
In such IPVD systems, material is sputtered from a target, which is charged negatively with respect to the plasma, usually by means of a DC power supply. The target is often of a planar magnetron design incorporating a magnetic circuit or other magnet structure which confines a plasma over the target for sputtering the target. The material from the target arrives at a wafer supported on a wafer support or table to which a bias is typically applied, often by means of an RF power supply connected to the substrate support through an impedance matching network.
A somewhat different IPVD geometry employs a plasma generated by a coil placed internal to the vacuum chamber. Such a system does not require dielectric chamber walls nor special shields to protect the dielectric walls. Such a system is described by Barnes et al. in U.S. Pat. No. 5,178,739, expressly incorporated by reference herein. Systems with coils outside of the chamber, as well as the system disclosed in the Barnes et al. patent, involve the use of inductive coils or other coupling elements, either inside or external to the vacuum, that are physically positioned and occupy space between the planes of the sputtering target and the wafer.
Whether a coupling element such as a coil is provided inside or outside of a vacuum chamber, dimensions of the system have been constrained by the need for adequate source-to-substrate separation to allow for the installation of the RF energy coupling elements between the source and the substrate. Adequate diameter must also be available around the wafer for installation of coils or other coupling elements. As a direct result of the increased source-to-substrate spacing due to the need to allow space for the coupling element, it is difficult to achieve adequate uniformity of deposition with such systems. If the height of the chamber is reduced to improve uniformity there is a loss of plasma density in the central region of the chamber and the percentage ionization of the coating material is reduced. Also, in practice, the entire system must fit within a constrained volume. As a result, there are frequently problems due to heating arising from the proximity of the RF coils to walls and other metal surfaces, which may necessitate extra cooling, which increases engineering and production costs and wastes power.
An IPVD apparatus with the coil in the chamber has the additional disadvantage that the coils are eroded by the plasma and must therefore consist of target grade material of the same type as that being sputtered from the target. Moreover, considerable cooling of coils placed in the vacuum chamber is needed. If liquid is used for this cooling of the coils, there is danger that the coils will be penetrated by uneven erosion or by arcing, causing a resulting leak of liquid coolant into the system, which is highly undesirable and will likely result in a long period of cleaning and requalification of the system. Furthermore, an excitation coil in the chamber also couples capacitively to the plasma, leading to inefficient use of the excitation power and to the broadening of the ion energy spectrum, which may have undesirable effects on the deposition process.
As a result of many of the above considerations, a method and an apparatus for ionized physical vapor deposition have been proposed by Drewery and Licata in the commonly assigned and copending U.S. Pat. No. 6,080,287 hereby expressly incorporated by reference herein. The method and the apparatus described in the Drewery et al. application provides for the efficient coupling of energy into the dense plasma by which coating material is ionized in IPVD processing systems, and does so without interfering with the optimum geometry of the chamber and without placing a coil or other coupling element into the vacuum chamber. It uses an apparatus provided with a ring-shaped source of coating material such as an annular sputtering target having at its center a coupling element such as a flat coil behind a dielectric window for coupling RF energy into the chamber to produce a high density reactively coupled plasma such as an inductively coupled plasma (ICP).
IPVD systems and methods such as those described above benefit from a high plasma density and plasma uniformity to efficiently achieve their objectives, particularly the objective of coating sub-micron high aspect ratio features. Higher plasma density leads to increased metal ionization. Improved plasma uniformity also reduces the effects of bias, for example RF bias produced by an RF source, on deposition uniformity and widens the acceptable range of other process parameters. Various of the systems described above are prone to loss of effectiveness due to a loss of ions to the walls of the chamber and as a result improving plasma uniformity.
In plasma systems of the types that do not involve IPVD, various techniques have been tried for reducing the loss of ions to the chamber walls. In low temperature plasma generation, for example, one approach to improving plasma uniformity and increase plasma density is the use of a "magnetic bucket". Magnetic fields of the type produced by magnetic buckets reduce the electron flux to the walls of a processing chamber by a type of confinement called magnetic mirror confinement. The tendency of the plasma to stay neutral, that is to have the same number of electrons as positive ions, reduces the ion flux to the walls as well. The different process chamber configurations of the systems discussed above each present problems in achieving plasma density and uniformity. Magnetic mirrors and magnetic buckets interact adversely with magnetron fields and degrade target utilization. This has made them less suitable for cases where high plasma density and uniformity are required, such as for IPVD.
For these and other reasons, there is a need to improve the deposition uniformity and increase plasma density in IPVD systems, including those with ICP plasma sources situated at the center of an annular target.